When providing information such as voice guidance and the like services to passengers sitting in seats inside an aircraft, a railcar and the like filled with loud noise, the noise in the seats becomes a problem.
An interior space confined within a continuous wall like that of an aircraft or a car has a kind of sealed structure, and the noise environment remains persistent for the passengers when there is a noise source either inside or outside the space. Depending on a level of the noise, it can become a factor of physical and mental stresses to the passengers, and it therefore impairs the comfort. The noise thus poses a serious problem in the quality of service especially when the service is provided for the passengers inside a cabin in an aircraft and the like.
In the case of an aircraft, the noise of apparatuses such as propellers and engines for generating thrust of the aircraft and the sound associated with airflow produced around the airframe moving in the aerial space such as a wind noise during the flight are main sources of the noise. Since the noise inside the cabin interferes with the voice guidance and the like services in addition to causing discomfort of the passengers, an improvement of it is strongly desired.
To this end, the methods hitherto used are passive damping means in most instances, such that a sound insulation material having sound absorbing properties like sound barrier material and acoustic absorbing material is disposed between the sealed structure and the noise source as the measures to reduce the noise inside the sealed cabin. A high-density barrier material and a sound-absorbing sheet are examples used as the sound barrier material and the acoustic absorbing material respectively. Any material of sound absorbing property generally has a high density, which accompanies an extra weight. The fuel consumption increases with increase in weight, and the cruising range decreases. It hence causes a decline in economical efficiency and deterioration of the function as an aircraft. There are also other aspects of functionality such as decrease in the strength as being susceptible to damages and the design of esthetical quality that are not ignorable as the structural materials.
To address the problems associated with the noise reduction measures using the passive damping means discussed above, there exists an idea disclosed recently of a noise reduction device capable of executing control for reduction of noise coming to a control point by selectively operating any of a fixed filter and an adaptive filter and outputting a control sound of opposite phase to that of the noise coming to the control point (known as active noise control, refer to Patent Literature 1, for example).
In this example, the noise reduction device comprises noise-detecting microphone 9101, adaptive filter 9201, control speaker 9401, error microphone 9501 and fixed filter 9601, as shown in FIG. 9, and the device executes the noise control by selecting adaptive filter 9201 when a change occurs in the condition of noise. Noise-detecting microphone 9101 detects a noise and outputs it as a noise signal to adaptive filter 9201. Adaptive filter 9201 executes signal processing on the noise signal by using a filter coefficient and generates a control signal, and control speaker 9401 outputs a control sound toward the control point. Error microphone 9501 is placed in the control point for detecting a difference between the noise and the control sound, and output it as an error signal. Adaptive filter 9201 executes an updating process of the filter coefficient in a manner to minimize the error signal by using the Filtered-X_LMS algorism, for instance, to update the filter coefficient in response to a change in the condition of the noise, and generates a new control signal. When the filter coefficient being updated converges, the device carries out the noise control by selecting fixed filter 9601, which is preset with the converged filter coefficient.
The above method may make the noise-detecting microphone and the error microphone unable to continue their normal detecting functions if there is any obstacle such as a pillow, blanket, book and the like object placed around the microphones, thereby leaving a problem of presenting a possibility of increasing the noise in the control point depending on a condition of such obstacle. Among some measures known to address this problem, there are techniques disclosed as conventional arts, one of which optimizes the control by changing the filter coefficient according to a distance to the obstacle if any, and the other suppresses an abnormal sound attributed to the obstacle by setting a threshold for the filter coefficient (refer to Patent Literatures 2 and 3, for example).
According to the above conventional arts, however, the devices are designed to calculate the filter coefficient continuously in real time, and hence there has remained a possibility depending on the condition of the obstacle that the noise becomes larger than that without carrying out the control for noise reduction, and it discomforts the passengers when a control sound updated according to the calculated filter coefficient is output immediately. Moreover, the above arts have not given any consideration to operation with the noise-detecting microphone covered with an obstacle, thereby making it unlikely to achieve a steady effect of the noise reduction.